DISEASES OP VEGETABLES. 



135 



aud cracks. The wound that the stem thus re- 

 ceives cicatrizes hefore the petiola separates, and 

 the petiola separates at last in consequence of the 

 interrupted connection between the leaf and stem 

 which the crack has occasioned. This, it must 

 be confessed, does not make up for the deficiencies 

 of the hypothesis of Du Hamel ; for in the first 

 place, there is no proof that the bond of union 

 between the leaf and stem cracks in the manner 

 here supposed ; and even upon tlie supposition 

 of its being the fact, it is, in the second place, 

 extremely improbable that the petiola should, 

 after the crocking of this bond of union, still con- 

 tinue attached to the stem till the wound thus 

 occasioned has cicatrized, because, when the ori- 

 ginal bond of union cracks, there remains no 

 other attachment by which the petiola is to re- 

 tain its hold. WiMenow quotes another expla- 

 nation by Vorlick : The leaf, which possesses a 

 peculiar vitality within itself, though dependent 

 upon the vitality of the plant, and generally of 

 shorter duration, dies when it reaches maturity ; 

 and the plant being able to exist, for a time, 

 without leaves, throws off the dead leaf, as the 

 animal throws off the dead part from the sound 

 part. But the peculiar vitality which the leaf 

 is here supposed to possess seems to me, says Dr 

 Keith, to be altogether a groundless assumption, 

 and an unphilosophical multiplication of causes 

 without any apparent necessity. Is it not rather 

 the individual vitality of the plant extended to a 

 perishable organ, and again withdrawn when that 

 organ has discharged its destined functions, or 

 become, by disease or decay, unfit for the pur- 

 poses of vegetation ? This is perhaps a better 

 founded supposition than the former, though the 

 reference to the phenomenon of tlie throwing off 

 of the dead part from the sound in the animal sub- 

 ject, is sufficiently well adapted to the purposes 

 of illustration, and the analogy sufficiently strik- 

 ing, at least under some of its aspects, to warrant 

 its introduction ; for which, or for similar reasons. 

 Sir J. E. Smith gives his sanction to the opinion 

 of Vorlick, which he had himself indeed been pre- 

 viously led to adopt, though he was anticipated in 

 the pubUcation. The notion was first suggested to 

 him by some remarks of Mr Fairburn of Chel- 

 sea, who had observed that, in the transplanting 

 of trees, if the injury extends suddenly beyond 

 the leaf, then the leaf remains firmly attached to 

 the twig, even though dead ; but when the leaves 

 alone are affected, and the vital energy acting 

 with full force in the branch, the leaves are 

 thrown off, or fall on the slightest touch ; hence 

 Sir J. E. Smith concludes that leaves are thrown 

 off by a process similar to that of the sloughing 

 of discarded parts in the animal economy. It 

 does not, however, seem quite evident to me, 

 continues Dr Keith, that the idea of sloughing is 

 comprehended in the opinion of Vorlick. Slough- 

 ing in the animal economy is the exertion of that 



power by which the vital energy is capable of 

 throwing off a part that has accidentally become 

 diseased, and unfit for discharging the functions 

 to which it was originally destined, but not that 

 power by which it is capable of throwing off a 

 distinct organ, intended by nature to be finally 

 separated from the individual. Now, in the case 

 of the disfoliation of the plant, there is, for the 

 most part, no disease, but merely a gradual and 

 natural decay, which reduces the leaf to a state 

 indeed no longer fit for the purposes of vegeta- 

 tion, but to which it was intended by nature to 

 be reduced, for the purpose of facilitating its se- 

 paration from the plant ; and hence it alwaj'S 

 separates in a determinate manner, and at a de- 

 terminate point, namely, at the base of the foot 

 stalk, which forms as it were a sort of natural 

 joint, to which there is nothing analogous in the 

 case of sloughing. If this were not the fact, it 

 might be expected that a part of a leaf, or even 

 the whole of it, should occasionally become per- 

 manent as well as the branches, though no such 

 thing has ever yet happened. In the sloughing 

 of the diseased part there is yet another circum- 

 stance coinciding with the analogy that is here 

 instituted. The part supplying the place of the 

 slough, on throwing it off, is formed, or exists 

 ah'eady formed, immediately beneath it, and is 

 precisely of the same character with what the 

 slough originally was, which slough it pushes 

 off as it comes itself to maturity, or acquires 

 strength sufficient for the effort. But the leaves 

 fall off, -when they have reached maturity, of 

 their own accord, without being at all pushed 

 off by the new ones, which are yet merely in 

 embryo, and do not even occupy the place of the 

 old leaves, but are only formed contiguous to 

 them, except in the case of the plane tree, the 

 new leaf of which is formed precisely under the 

 base of the foot stalk of the old leaf; and yet we 

 would not call the fall of that leaf sloughing, be- 

 cause the new leaf does not after all push off the 

 old one, and because there is here, as in other 

 cases, the same natural articulation uniting the 

 leaf to the branch or stem, and rendering it a 

 distinct organ, that is ultimately and spontane- 

 ously to detach itself from the plant. Not that 

 there exists no example whatever of vegetable 

 sloughing, which the same tree wiU also furnish, 

 in the annual, or rather continual, exfoliation of 

 its bark ; but that the fall of the leaf does not 

 seem to afford that example. I can see an ob- 

 jection, adds Dr Keith, that may be urged against 

 the above argument, from the fact of the slough- 

 ing of the actual skin of the snake and other 

 species of serpents, which may be regarded as a 

 distinct organ. But although the skin of the 

 snake, or of any other animal, may be regarded 

 as a distinct organ, yet it must be in a light very 

 different from that of an organ attached to the 

 body of a plant or animal by a natural joint or 



